Most gases will condense into a liquid form if the temperature of the gas is reduced sufficiently. However, the temperature at which gas to liquid condensation takes place at atmospheric pressure is often very low, for example, at atmospheric pressure propane gas condenses to a liquid at −43° C.
The temperature at which condensation occurs can be increased by increasing the pressure of the gas. Some gases, such as propane and butane, for example, can be easily condensed into a liquid form by compressing the gas. Such compressed gases are often known as volatile liquids, because they will vaporize to form a gaseous state as soon as the pressure is reduced. Therefore, in order to maintain the compressed gas in the liquid form the pressure must be maintained. This is usually achieved by storing the volatile liquid in pressurized containers.
To allow for removal of the gas from the pressurised container the container is usually fitted with a tap arrangement. When the tap is opened, the pressure in the container is reduced and the volatile liquid begins to vaporize. The vapor exits the container via the tap and will continue to do so until the tap is closed. After the tap is closed, the volatile liquid will continue to vaporize until the pressure inside the container becomes sufficient to maintain the gas in the liquid state.
As the volatile liquid vaporizes, energy is drawn from the body of the liquid to fuel the vaporization process. Therefore, the temperature of the body of liquid will fall as the liquid vaporizes.
A problem associated with pressurized containers of volatile liquids is that it is generally difficult to detect the level of liquid left in the container. Therefore, it is difficult to ascertain when the container will become emptied. This is a particular problem in use of liquid propane and butane cylinders with, for example, gas powered grills, patio heaters, room heaters, refrigerators, cookers and the like. In such instances it is unusual for a consumer to have a spare fuel cylinder available, and it can be very inconvenient if the gas bottle becomes empty part way through cooking a meal or such like. A device is available for indicating the liquid level remaining in a propane gas bottle. The device comprises a flexible sheet in which a plurality of rows of temperature-sensitive sensors are embedded. The device contains a magnetic material so that it can be attached to the exterior sidewall of a gas bottle just above the base. In use, each sensor of the device senses the temperature of the bottle and the sheet changes color at the points where a temperature at or below a set temperature is detected. Thus the device is arranged to change color below the liquid level.
Whilst such a device is cheap to manufacture and easy to use, it can be adversely affected by external temperature changes, such as sunlight and/or wind or rain incident on the device. Furthermore, the device needs to be regularly observed.